The objective of project B4 is to study the influence of vibrational
energy on the chemical reactivity on surfaces as well as the energy
dissipation of electronic excitations into vibrational degrees of
freedom. Vibrational excitations are very important as part of the
thermal activation of chemical reactions. For this purpose we prepare
a high density of vibrationally excited adsorbates using an IR-picosecond
laser system. We study the population of vibrational excitations of
adsorbed molecules and its temporal evolution using sum frequency
generation.

Fig 1

Fig 2

In a first experiment, we studied the lifetime of the vibrational
excitation of CO on Si(100). An IR-laser pumps the molecules into
the vibrationally excited state (Fig. 1). Some variable time later
a IR-visible laser pulse pair probes the population in the vibrationally
excited state. The decay of the population with time yields directly
the vibrational life time. Surprisingly, we observed that the excitation
decays in approx. 2 ns (Fig. 2). This finding is puzzling as the vibrational
quantum of the CO mode (2046 cm-1) is four times as large as the largest
Si phonon energy (520 cm-1). Hence, four phonons would have to be
generate simultaneously. That is a very unlikely process. Hence, we
expected a life several orders of magnitude larger.

Sophisticated modelling is carried out in collaboration with project B7. This work suggests that overtone and combination mode states in the progression of the bending and shift modes serve as intermediates such that only one phonon needs to be excited. Thus, the decay proceeds more rapidly than expected at first glance.

Meanwhile, we have carried out similar studies for H adsorbed on Ge surfaces. The rather small energy of the largest phonon in Ge (310 cm-1) was considered as a reason why the vibrational life time may be longer than for Si. It turn out that this is not the case.

Fig 3

In the future, we want to combine IR- and UV-excitations. An IR-laser
pulse prepares vibrationally excited molecules. We then hit these
molecules with a UV laser pulse (Fig. 3). The UV-excitation causes
photosorption of some adsorbate molecules. This process is believed
to be caused by the interaction with laser excited electrons generated
in the substrate. In the first experiments, we will study how the
photodesorption cross section of these adsorbates is enhanced by vibrational
excitation. CO adsorbed on Si(100) and NH3 adsorbed on Cu(111) are
the systems we started to look at.

In the second round of experiments, we will first initiate the photodesorption
process by a UV-picosecond laser pulse. Subsequently, we will probe
the resulting population of highly vibrationally excited molecules
using sum-frequency generation spectroscopy. Since the lifetime of
electronic excitations of adsorbates is extremely short (several fs),
always only a small fraction of the excited molecules desorbs, whereas
the majority is recaptured by the surface due to premature quenching.
We expect these experiments to provide novel insights into the energy
exchange between electronic and vibrational degrees of freedom of
adsorbed molecules.